Viscosity and nonlinearity effects on the forces and waves generated by a floating twin hull under heave oscillation

Nonlinear hydrodynamics of a twin rectangular hull under heave oscillation is analyzed using numerical methods. Two-dimensional nonlinear time-domain solutions to both inviscid and viscous problems are obtained and the results are compared with linear, inviscid frequency-domain results obtained in [26] to quantify nonlinear and viscous effects. Finite-difference methods based on boundary-fitted coordinates are used for solving the governing equations in the time domain [2]. A primitive-variables based projection method [6] is used for the viscous analysis and a mixed Eulerian–Lagrangian formulation [11] for inviscid analysis. The algorithms are validated and the order of accuracy determined by comparing the results obtained from the present algorithm with the experimental results of Vugt [22] for a heaving rectangle in the free surface. The present study on the twin-hull hydrodynamics shows that at large and non-resonant regular frequencies, and small amplitude of body oscillation, the fluid viscosity does not significantly affect the wave motion and the radiation forces. At low frequencies however the viscosity effect is found to be significant even for small amplitude of body oscillation. In particular, the hydrodynamic force obtained from the nonlinear viscous analysis is found to be closer to the linear inviscid force than the nonlinear inviscid force to the linear inviscid force, the reason for which is attributed to the wave dampening effect of viscosity. Since the wave lengths generated at smaller frequencies of oscillation are longer and therefore the waves could have a more significant effect on the dynamic pressure on the bottom of the hulls which contribute to the heave force, the correlation between the heave force and the wave elevation is found to be larger at smaller frequencies. Because of nonlinearity, the wave radiation and wave damping force remained nonzero even at and around the resonant frequencies – with the resonant frequencies as determined in [26] using linear potential flow theory. As to be expected, the nonlinear effect on the wave force is found to be significant at all frequencies for large amplitude of oscillation compared to the hull draft. The effect of viscosity on the force, by flow separation, is also found to be significant for large amplitude of body oscillation.     

Estimation of scour around submarine pipelines with Artificial Neural Network

The process of scour around submarine pipelines laid on mobile beds is complicated due to physical processes arising from the triple interaction of waves/currents, beds and pipelines. This paper presents Artificial Neural Network (ANN) models for predicting the scour depth beneath submarine pipelines for different storm conditions. The storm conditions are considered for both regular and irregular wave attacks. The developed models use the Feed Forward Back Propagation (FFBP) Artificial Neural Network (ANN) technique. The training, validation and testing data are selected from appropriate experimental data collected in this study. Various estimation models were developed using both deep water wave parameters and local wave parameters. Alternative ANN models with different inputs and neuron numbers were evaluated by determining the best models using a trial and error approach. The estimation results show good agreement with measurements.     

近54年京津冀地区热浪时空变化特征及影响因素

基于1960—2013年京津冀及周边地区34个气象站逐日最高气温和相对湿度资料,利用高温热浪模型,辅以趋势分析、突变检验及相关分析等方法,研究近54年京津冀地区热浪时空变化特征,探讨城市化对热浪变化的影响,并尝试寻找对热浪异常具有稳定指示意义的环流因子。结果表明:1960—2013年京津冀地区热浪变化具有明显的阶段性,以20世纪70年代中期为转折,热浪呈先减少后增加趋势;京津冀地区热浪空间格局变化整体呈南减北增,东南平原区热浪呈下降趋势,北部生态涵养区呈现增加趋势;在区域尺度上,城市化或迁站影响并未改变北京极端热浪变化趋势,主要影响以轻度和中度热浪变化为主;西太平洋副热带高压和青藏高原反气旋环流与京津冀地区热浪异常关系最为显著,对热浪异常是一种稳定且强烈的指示信号。当青藏高原高空反气旋环流异常偏强,西太平洋副热带高压明显偏北,京津冀地区发生超级热浪可能性较大。     

适用于曲边界的浅水非线性波浪数值模型

实际工程中存在大量的曲边界,因此在曲边界上的计算准确性可以考察出一个数值模型的实用价值。利用Beji的改进型Boussinesq方程建立了一个有限元方法的数值波浪模型。造波方面采用Fenton提出的非线性规则波浪解;在墙边界处,以求解法线方向和切线方向的速度和导数代替求解x、y方向的速度和导数,从而使边界条件直接适用、严格满足,保证了对曲边界计算的准确性。"重开始广义极小残量法"的使用保证了求解方程组的效率和精度,使造波和边界处理方法的有效性和准确性得到了合理地诠释。通过与试验数据、他人数值结果、解析解的比对,显示出该模型计算稳定、结果准确,真正体现出了有限元方法对曲边界适用的优势。     

Life Geography from the Child's Point of View

Abstract

A second look is taken at “The Geography of the U.S. in the Year 2,000.” A new interplay is seen of the countervailing pressures to disperse and to agglomerate. Polynucleated urban regions are seen, organized within and around a global poly center. There are certain imperatives: those of demographic cycles and of the economic long wave. These will interact with information-age technologies to change family structures, life styles, and locational preferences. The unexpected also should be expected: “catastrophes” in which existing arrangements are transformed and new structures put into place to replace them. A key to understanding is to continually probe the second derivatives: change in the nature of change.     

Static scalar-tensor universes and gravitational waves’ amplification

We study several static models of the Universe, along with scalar-tensor gravity. Lambda-Universes arise naturally in this framework.      

Nonlinear field line resonances. Effect of Hall term on plasma compression: 1D Hall-MHD modeling

A model is presented that describes the excitation of small-scale density perturbations and electromagnetic fields by standing shear Alfvén waves in a Cartesian geometry. The model includes the effects of plasma betas and the Hall term effects. The characteristics of magnetospheric density cavities and the formation of the significant peak in density are discussed in the paper.     

Sound waves in the intracluster medium

We present an analysis of the behaviour of a perturbed radio cocoon. Comparisons with observations of sound waves detected in the Perseus and Virgo clusters suggest the separations of observed ripples correspond to the natural oscillation frequency of the cocoon. An energy injection rate consistent with active galactic nucleus power is required to offset the strong acoustic damping of cocoon oscillations, suggesting the sources are in equilibrium with the intracluster medium (ICM), and the oscillations are effectively undamped. Viscous dissipation of sound waves provides ICM heating that can quench cooling flows on time-scales greatly exceeding the oscillation time-scale. Thermal conductivity is likely to be heavily suppressed.     

Accretion flow behaviour during the evolution of the quasi-periodic oscillation frequency of XTE J1550−564 in 1998 outburst

Low and intermediate frequency quasi-periodic oscillations (QPOs) are thought to be due to oscillations of Comptonizing regions or hot regions embedded in Keplerian discs. Observational evidence of evolutions of QPOs would therefore be very important as they throw lights on the dynamics of the hotter region. Our aim is to find systems in which there is a well-defined correlation among the frequencies of the QPOs over a range of time so as to understand the physical picture. In this paper, we concentrate on the archival data of XTE J1550−564 obtained during 1998 outburst, and study the systematic drifts during the rising phase from the 1998 September 7 to the 1998 September 19, when the QPO frequency increased monotonically from 81 mHz to 13.1 Hz. Immediately after that, QPO frequency started to decrease and on the 1998 September 26, the QPO frequency became 2.62 Hz. After that, its value remained almost constant. This frequency drift can be modelled satisfactorily with a propagatory oscillating shock solution where the post-shock region behaves as the Comptonized region. Comparing with the nature of a more recent 2005 outburst of another black hole candidate GRO 1655−40, where QPOs disappeared at the end of the rising phase, we conjecture that this so-called 'outburst' may not be a full-fledged outburst.     

Gravitational waves from resolvable massive black hole binary systems and observations with Pulsar Timing Arrays

Massive black holes are key components of the assembly and evolution of cosmic structures, and a number of surveys are currently on going or planned to probe the demographics of these objects and to gain insight into the relevant physical processes. Pulsar Timing Arrays (PTAs) currently provide the only means to observe gravitational radiation from massive black hole binary systems with masses  ≳10⁷ M_⊙  . The whole cosmic population produces a stochastic background that could be detectable with upcoming PTAs. Sources sufficiently close and/or massive generate gravitational radiation that significantly exceeds the level of the background and could be individually resolved. We consider a wide range of massive black hole binary assembly scenarios, investigate the distribution of the main physical parameters of the sources, such as masses and redshift, and explore the consequences for PTAs observations. Depending on the specific massive black hole population model, we estimate that on average at least one resolvable source produces timing residuals in the range  ∼5–50 ns  . PTAs, and in particular the future Square Kilometre Array, can plausibly detect these unique systems, although the events are likely to be rare. These observations would naturally complement on the high-mass end of the massive black hole distribution function future surveys carried out by the Laser Interferometer Space Antenna .